Sounds like you need to go see the dealer. One word of caution if your dealer tells you to put in higher octane to fix a problem when your truck is designed to run on regular, it might be an indication of how you'll be treated down the line. Sounds like he was hoping to get rid of you.
If it's a brand new truck, he should have said bring it on in Monday morning and we'll take a look.
If you keep having problems with them, then look for another Nissan dealer. It's important to have a good working relationship with your service department. Also, drive down to the dealership and tell your salesman about the problem. He should lead you right over to the service department and have an appointment set up right away. It's hard to turn people away when they're right in front of you, and the salesman "should" be ready to make you happy.
Oh, and here are my guesses. Really guesses cause I'm not sure what your problem is. Fuel pump problem, or the spare isn't tightened to the truck well, or the leaf spring has a problem and is popping to relieve some tension. Just guesses though.
